1. Field of the Invention
The present invention generally relates to a method of washing a photomask used as an original plate in a photolithography step for fabricating a semiconductor device (LSI), and more particularly, it relates to an improved method of washing a photomask in an improved manner which permits obtaining a extremely clean surface. The present invention also relates to an apparatus for washing a photomask in an improved manner which permits obtaining a extremely clean surface. The present invention further relates to a washing solution capable of obtaining such a photomask having an extremely clean surface.
2. Description of the Prior Art
A photomask is used as an original plate when transferring the pattern of an integrated circuit onto a surface of a wafer with a transfer unit in a photolithography step for fabricating a semiconductor device. If the pattern formed on a surface of the photomask is defective or the photomask is foul with foreign matter exceeding the resolution limit, the defect or the foreign matter is disadvantageously transferred onto the wafer as part of the pattern. Therefore, no defect or foreign matter exceeding the resolution limit is allowed on the surface of the photomask. Due to high integration and refinement of the integrated circuit, the size of allowable defect or foreign matter is limited to not more than 0.5 μm.
In general, such a photomask is washed by a method based on RCA washing, employing a mixed solution of acid such as sulfuric acid and aqueous hydrogen peroxide and a mixed solution of an alkaline chemical solution such as aqueous ammonia and aqueous hydrogen peroxide, which has been field-proven in a wafer washing step.
FIG. 11 illustrates the flow of the conventional washing process.
In a step 1, the photomask is washed with a high-temperature mixed solution of sulfuric acid and aqueous hydrogen peroxide, in order to decompose organic matter such as resist or a solvent present on the surface of the photomask and remove metal impurities. The wettability of the mask surface is improved through this step to improve the efficiency of later washing.
Then, in a step 2, the photomask is rinsed with high purity water for removing chemicals such as sulfuric acid.
Then, in a step 3, the photomask is dipped in a tank and washed in a heated mixed solution of ammonia and aqueous hydrogen peroxide, in order to remove foreign matter adhering to the photomask. At this time, ultrasonic waves such as megasonic waves may be applied to the dipping tank.
Also after the step 3, the photomask must be sufficiently rinsed with high purity water in a step 4. Finally, the photomask rinsed with high purity water is dried in a step 5. At the step 3, the photomask may be washed not with the mixed solution of ammonia and aqueous hydrogen peroxide but with only high purity water or high purity water mixed with a detergent, with application of ultrasonic waves such as megasonic waves.
In the aforementioned dipping system, the throughput can be improved by simultaneously dipping a plurality of photomasks in one tank, while a remarkably contaminated photomask may disadvantageously contaminate other relatively clean photomasks.
In order to improve this point, a spin washing method of horizontally rotating a photomask and applying a chemical solution, high purity water or the like thereto from a fixed or swinging nozzle is performed as a system throwing away the chemical solution for washing a single photomask. In the spin system, the photomask may be subjected to mechanical washing such as high-pressure high purity water jet rinsing, megasonic high purity water rinsing or the like for further effectively removing foreign matter.
The aforementioned step 3 of treating the photomask with aqueous ammonia/aqueous hydrogen peroxide for removing foreign matter has the following problem: In the dip washing, a plurality of photomasks are treated with the same chemical solution, and hence the chemical solution must be frequently renewed to avoid its deterioration or contamination, leading to increase of the amount of the chemical solution consumed. If the washing efficiency (washing yield) is inferior, each photomask is washed a plurality of times, to result in increase of the amounts of chemicals or high purity water and energy such as electricity.
A phase-shift photomask improving the resolution of resist on a wafer by partially shifting the phase of light transmitted through the photomask has recently been developed and put into practice. An MoSiON film is used as a material of a shading mask for a halftone photomask, which is a kind of such phase-shift photomask.
However, the transmittance and the phase angle of the MoSiON film remarkably fluctuate through washing with an alkaline chemical such as the conventional dipping in aqueous ammonia/aqueous hydrogen peroxide. Thus, the MoSiON film cannot keep its quality when the photomask is shipped as a product. Therefore, washing with aqueous ammonia/aqueous hydrogen peroxide effective for removing foreign matter cannot be applied to the MoSiON film, which is in practice washed with only high purity water or a detergent. Thus, foreign matter disadvantageously remains on the MoSiON film.